Python gives me NameError during listcomp, but not if done as part of regular loops

Question

In python I'm having an issue with running some code, but only when it's part of a list comprehension.

The following code works:

from os import listdir
from os.path import isfile, join

IMPORT_DIR = '/media/import_files/'
files = listdir(IMPORT_DIR)

FILES_FOR_IMPORT = []
for f in files:
    if isfile(join(IMPORT_DIR, f)):
        FILES_FOR_IMPORT.append(f)

However, the following (supposedly equivalent) list comprehension throws an error:

FILES_FOR_IMPORT = [ f for f in listdir(IMPORT_DIR) if isfile(join(IMPORT_DIR,f)) ]

Stack trace:

  File "/home/mitch/workspace/mysite/flat_file_importer/ImporterMain.py", line 12, in <module>
    class ImporterMain():
  File "/home/mitch/workspace/mysite/flat_file_importer/ImporterMain.py", line 25, in ImporterMain
    FILES_FOR_IMPORT = [ f for f in listdir(IMPORT_DIR) if isfile(join(IMPORT_DIR,f)) ]
  File "/home/mitch/workspace/mysite/flat_file_importer/ImporterMain.py", line 25, in <listcomp>
    FILES_FOR_IMPORT = [ f for f in listdir(IMPORT_DIR) if isfile(join(IMPORT_DIR,f)) ]
NameError: name 'IMPORT_DIR' is not defined

In testing, I have the first bit before the second bit; both are getting executed. The first works, the second does not. Same behavior if I only execute one at a time - one works, one does not.

Clearly the IMPORT_DIR variable is defined - it is being used in the first bit of code, so I'm not sure why Python is spitting this error at me.

The list comp, which I've used before, is stolen from here: https://stackoverflow.com/a/3207973/3665278

The full code (with the bit that is working first and the list comp second):

from os import listdir
from os.path import isfile, join
import re


class ImporterMain():
    IMPORT_DIR = '/media/import_files/'

    files = listdir(IMPORT_DIR)

    FILES_FOR_IMPORT = []
    for f in files:
        if isfile(join(IMPORT_DIR, f)):
            FILES_FOR_IMPORT.append(f)

    FILES_FOR_IMPORT = [ f for f in listdir(IMPORT_DIR) if isfile(join(IMPORT_DIR,f)) ]

Is this Python 3.x ? And is the list comprehension inside a class? From the stack trace it seems so. — Anand S Kumar, Aug 05 '15 at 14:31
Your `IMPORT_DIR` name is a *class attribute*, judging by the traceback. See the duplicate as to what is going on. — Martijn Pieters, Aug 05 '15 at 14:32
@AnandSKumar, yes it is. I've updated the question with the full code, but it looks like Martijn has the answer — Mitch, Aug 05 '15 at 14:36

Python gives me NameError during listcomp, but not if done as part of regular loops

0 Answers0